Permanent waving is a process whereby a reducing agent is applied to the hair structure to open the disulfide linkages of the hair which are formed by the amino acid cystine. In the process, hair is wound onto an appropriate mandrel, e.g. roller or rod etc., prior to and/or following reduction with a suitable reducing agent. The protein chains flow under tension to assume the imparted shape. After rinsing, an oxidizing agent is then applied to re-establish or close the disulfide linkages which, in effect, hardens the protein structure to lock it into the new position.
Permanent waving may utilize a variety of reducing agents in a first step over a wide pH range. Depending upon pH, the waving process can be carried over a wide temperature range.
An essential element of a permanent waving composition is the reducing agent. Among the reducing agents are thioglycolic acid, salts, and esters thereof; thiolactic acid and salts thereof; alkali sulfides; alkali bisulfites, cysteine, and the like. The bulk of the reducing compositions are based on thioglycolic acid, salts, or esters thereof.
The hair may be waved with thioglycolates under acid conditions, where the pH will range from about 5.0 to about 6.9, preferably 6.5 to about 6.9. To this end, citric, lactic, phosphoric, and weak carboxylic acids are used as common acidifying agents. Acid waves based on thioglycolates utilize elevated temperatures up to about 140.degree. F. using heat caps and hair dryers. Alkali bisulfite and bisulfide waves are also acid, namely at a pH from about 5.5 to 6.9, and are applied at room temperature.
Alkaline or "cold" wves are used at a pH in the range of 7.5 to 10.0 with ammonia, alkali carbonates and bicarbonates, ethanolamines, and alkali phosphates used as common alkalizing agents. Alkaline waves are also applied at room temperature.
The concentration of reducing agent, pH, and temperature are dictated by the hair condition, the time of processing desired, and the desired tightness of the curl.
Contact with the reducing agent may range from 10 minutes or less to 30 minutes or more. After an appropriate time with contact with the reducing agent, the reducing agent is rinsed from the hair, and an oxidizing agent is applied to close the disulfide bonds and set the hair. Excess oxidizing agent is then rinsed from the hair, and the hair dried. The most common oxidizing agents are hydrogen peroxide and bromate salts. Peroxides are applied over a pH range from 2.5 to about 4.0 and bromates from a pH of about 6.0 to about 8.0. Application is at ambient or elevated temperatures.
Independent of the type of permanent wave applied, moisture is deleterious to the appearance of a permanent wave. A substantial increase in moisture content of the hair over that which existed at the time the wave was applied will tend to cause the curl to drop and become limp. By contrast, when hair of desired moisture content is exposed to low relative humidities, the hair tends to lose moisture and become frizzy.